1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus for lighting a discharge lamp, in particular, a high-luminance discharge lamp such as a high-pressure mercury lamp, a metal halide lamp, or a xenon lamp.
2. Description of the Related Art
In general, a discharge lamp reaches a steady state (steady lighting) through a “breakdown phase”, a “glow discharge phase”, and an “arc discharge phase”. Examples of an apparatus performing such lighting control of a discharge lamp include an apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2011-29012.
It is disclosed in Japanese Unexamined Patent Application Publication No. 2011-29012 that the following start-up sequence is effective to expedite the dissipation of the state of an asymmetrical discharge, thereby realizing reliable lighting in the “arc discharge phase” at the start-up time of the discharge lamp.    (1) First, at the time of initiating the start-up process of a discharge lamp, as a result of an inverter being driven at the third subharmonic frequency (fo/3) of the resonant frequency of a resonant circuit, a feeding circuit outputs a no-load open voltage.    (2) When the discharge lamp starts discharging due to dielectric breakdown, the driving frequency of the inverter is gradually decreased from the third subharmonic frequency (fo/3) of the resonant circuit to a predetermined threshold frequency.    (3) After the threshold frequency has been reached, the driving frequency of the inverter is switched to a stable-lighting frequency.
In the discharge lamp lighting apparatus described above, while the driving frequency of an inverter is decreased from fo/3 to a first threshold frequency, odd-order subharmonics such as fo/5 (fifth order subharmonic) and fo/7 (seventh order subharmonic) may be included. A resonant circuit connected to an inverter circuit generates a high voltage at such odd-order subharmonic frequencies and, hence, the state of a discharge lamp varies while the driving frequency of the inverter circuit is decreased from fo/3 to the first threshold frequency. For example, immediately after transition to an arc discharge state, the discharge state is not stable, depending on the state of a discharge material, before the whole discharge material sealed in the lamp is vaporized. When an arc discharge can no longer be maintained, the lamp has a tendency to return to a glow discharge state. However, when a voltage high sufficient to maintain the glow discharge state cannot be immediately supplied to the discharge lamp, a no-load state is entered, whereby “dying out” occurs. When such “dying out” occurs, a no-control state may be entered, and an overvoltage may be generated in the resonant circuit. This may result in the withstand voltage failure of the resonant circuit in the output stage, characteristics degradation due to the heat generation of switching devices in the inverter circuit, and a reduction in the lifetime of the discharge lamp due to damage.